The exhaust gases of internal combustion engines contain pollutants such as hydrocarbons, carbon monoxide and nitrogen oxides (NOx) that foul the air. Emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants have been set by various governments and must be met by older as well as new vehicles. In order to meet such standards, catalytic converters containing a three way catalyst (TWC) may be located in the exhaust gas line of internal combustion engines. The use of exhaust gas catalysts have contributed to a significant improvement in air quality. The TWC is the most commonly used catalyst and it provides the three functions of oxidation of CO, oxidation of unburned hydrocarbons (HC's) and reduction of NOx to N2. TWCs typically utilize one or more platinum group metals (PGM) to simultaneously oxidize CO and HC and reduce NOx compounds. The most common catalytic components of a TWC are platinum (Pt), rhodium (Rh) and palladium (Pd).
The platinum group metals (PGM) in the TWC catalysts (e.g., platinum, palladium, rhodium, ruthenium and iridium) are typically dispersed on a high surface area, refractory metal oxide support, e.g., a high surface area alumina coating, or on an oxygen storage component (OSC), or their mixtures. The support is carried on a suitable substrate such as a monolithic substrate comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material. The TWC catalyst substrate may also be a wire mesh, typically a metal wire mesh, which is particularly useful in small engines.
Many TWC catalysts are manufactured with at least two separate catalyst coating compositions (washcoats) that are applied in the form of aqueous dispersions as successive layers on a substrate (for example, a honeycomb body composed of ceramic or metal) in order to separate noble metals, such as, palladium and rhodium which represent the main catalytically active species. Separation has been necessary historically because some platinum group metals, such as palladium and rhodium, can form an alloy which is known to be less catalytically active.
There is a need to provide single washcoat compositions containing precious metals (i.e., palladium and rhodium) while maintaining and/or improving catalytic performance as compared to compositions that provide these metals individually for separate layers. There is also a need for applying the single washcoat composition in one coating step. There is also continuing need to provide a TWC catalyst composites that utilize precious metals efficiently and remain effective to meet regulated HC, NOx, and CO conversions.